Modern digital subscriber line (DSL) technologies, such as asymmetric digital subscriber lines (ADSL) and very high speed digital subscriber lines (VDSL), create communication systems that provide bi-directional, high-speed data transmissions over twisted-pair phone lines. In DSL communication systems, both downstream and upstream signals travel on the same pair of wires and are separated from each other using a duplexing method. Currently, many DSL communication systems are configured to implement frequency division duplexing (FDD) in order to separate the downstream signals from the upstream signals. For FDD based DSL communication systems, the upstream and downstream signals occupy different frequency bands and are separated by transceivers using filters. However, FDD based DSL communication systems may suffer drawbacks, such as longer loop lengths, susceptibility to noise (e.g. crosstalk) at higher frequencies, and inefficiencies when managing asymmetry transmissions. Therefore, as the demand for faster transmission rates and shorter loop lengths continue to increase, the telecommunication industry has been considering other transmission schemes to manage DSL communication.
One alternative is to utilize synchronous time division duplexing (TDD) to transmit and receive upstream and downstream signals within a DSL communication system. TDD synchronizes the upstream and downstream communication periods to prevent overlapping of the upstream and downstream signals. For example, a synchronous TDD based DSL communication system may transmit downstream data and upstream data in an alternating fashion. Either downstream data or upstream data is transmitted for a certain time period, but both are not transmitted during the same time period. The downstream data and upstream data may be separated by guard time periods, which signify periods when no data is transmitted in either direction. Synchronous TDD based DSL communication systems may be more efficient at managing asymmetric transmissions between the upstream and downstream transmissions. For instance, within a synchronous TDD based DSL communication system, as the amount of data increases for an upstream transmission, communication capacity may be reallocated from the downstream transmission to the upstream transmission. Additionally, when the data traffic for the upstream transmission decreases, the synchronous TDD DSL communication system may reallocate the communication capacity back to the downstream transmission.
Even though synchronous TDD based DSL communication systems may be more efficient at managing asymmetric transmissions compared to FDD DSL communication systems, the current TDD frame structure is inherently static and causes a fixed nominal asymmetry ratio between downstream and upstream data rates. In other words, current TDD based DSL communication systems may allocate communication capacities (e.g. data rates) for upstream transmission and downstream transmission during the initialization state, but not during the “showtime” state. For example, after a connection is established between a transmitter and receiver for a DSL line and the transmitter is ready to transmit data to the receiver, (e.g. “showtime state”) the actual transmission of data within the DSL line may be lower than the allocated communication capacities. To compensate for the low data rate, the transmitter may add dummy bits or symbols to provide a constant transmission of data. The dummy bits or symbols may consume bandwidth within the DSL communication system without improving system performance. Furthermore, as bandwidth and data sampling rates continue to increase, the constant transmission of dummy bits or symbols may increase power consumption. Thus, a solution is needed to provide greater flexibility in managing DSL transmissions and reduces power consumption within synchronous TDD based DSL communication systems.